A high-sensitivity silver halide light-sensitive material has been long desired. In particular, a silver halide light-sensitive material which is spectrally sensitized has been demanded to have high sensitivity.
The spectral sensitization is a very important and essential technique for producing a high-sensitivity light-sensitive material excellent in color reproduction. The spectral sensitizer has such a function that it absorbs light in a long wavelength region which the silver halide photographic emulsion does not substantially absorb by nature and transmits the absorbed light energy to silver halide. Therefore, it is advantageous for elevating the photographic sensitivity to use a spectral sensitizer which increases the captured light amount. Accordingly, a large number of attempts have been made to increase the addition amount of the spectral sensitizer to a silver halide emulsion to thereby increase the captured light amount. However, if the spectral sensitizer is added to a silver halide emulsion in excess of the optimal amount, the sensitivity is rather greatly reduced. This is generally called dye desensitization which is, more specifically, a phenomenon that the sensitivity in the light-sensitive region inherent to silver halide, where the sensitizing dye does not substantially absorb light, is reduced. If the dye desensitization is large, the overall sensitivity becomes low though the spectral sensitization effect is provided. In other words, if the dye desensitization is reduced, the sensitivity in light absorption region by the sensitizing dye (namely, spectral sensitization sensitivity) is increased in proportion. Accordingly, it is a matter of very importance in the spectral sensitization technology to improve the dye desensitization. The dye desensitization is commonly greater as the sensitizing dye has light sensitivity in the longer wavelength region. This is described in T. H. James, The Theory of the Photographic Process, pp. 265-268, Macmillan (1966).
JP-A-47-28916 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-49-46738, JP-A-54-118236 and U.S. Pat. No. 4,011,083 describe methods for increasing the sensitivity while reducing the dye sensitization. However, according to the methods in these publications, the sensitizing dyes which can be used are restricted and the effect obtained is far from satisfaction. The most effective method for improving the dye desensitization known at present is the use in combination of a bisaminostilbene compound substituted by a pyrimidine derivative or a triazine derivative, which is described, for example, in JP-B-45-22189 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-54-18726, JP-A-52-4822, JP-A-52-151026 and U.S. Pat. No. 2,945,762. However, the sensitizing dye for which the combination use of the above-described compound is effective is a so-called M-band sensitization-type dye which shows a gentle sensitization peak, such as dicarbocyanine, tricarbocyanine, rhodacyanine and merocyanine and the use is restricted to the dyes having a sensitization peak in a relatively long wavelength region.
U.S. Pat. No. 3,695,888 describes that the sensitization in an infrared region can be obtained by the combination of tricarbocyanine with an ascorbic acid, British Patent 1,255,084 describes that the minus blue sensitivity is elevated by the combination use of a specific dye with an ascorbic acid, British Patent 1,064,193 describes that the sensitivity can be increased by the combination use of a specific dye with an ascorbic acid and U.S. Pat. No. 3,809,561 describes a combination use of a desensitive nucleus-containing cyanine dye with a supersensitizer such as an ascorbic acid.
However, according to the above-described conventional techniques, the sensitization effect by the dye is still not yet satisfactory and if the sensitization effect is large, it is likely accompanied by the increase in fog.
It is also known, as described in T. Tani, et al., Journal of the Physical Chemistry, Vol. 94, p. 1298 (1990), that the sensitizing dye having a reduction potential higher than -1.25 V is low in the relative quantum yield of spectral sensitization. In order to increase the relative quantum yield of spectral sensitization of the dye, supersensitization by trapping positive holes has been proposed, for example, in the above-described The Theory of the photographic Process, pp. 259-265 (1966). However, more effective supersensitizer has been demanded.
In order to achieve high sensitivity of a silver halide photographic material, investigations have been made from the old to effect reduction sensitization. For example, U.S. Pat. No. 2,487,850 discloses a tin compound, U.S. Pat. No. 2,512,925 discloses a polyamine compound and British Patent 789,823 discloses a thiourea dioxide-based compound as effective reduction sensitizers. Further, Photographic Science and Engineering, Vol. 23, p. 113 (1979) sets forth comparison on the properties of silver nuclei formed by various reduction sensitization methods and methods using dimethylamineborane, stannous chloride, hydrazine, high pH ripening or low pAg ripening are employed. The reduction sensitization method is also described in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777 and 3,930,867. JP-B-57-33572 and JP-B-58-1410 describe not only the selection of reduction sensitizers but also the design for reduction sensitization method.
However, according to the investigations by the present inventors, it is found that when a sensitizing dye is adsorbed to a silver halide grain having been subjected to reduction sensitization, the fog is increased. In order to prevent desorption (in particular, at a high humidity) of a sensitizing dye from a silver halide grain in the light-sensitive material, the sensitizing dye may be adsorbed at a high temperature (50.degree. C. or higher) but this operation also causes deterioration with respect to the fog. Further, a sensitizing dye may be adsorbed before chemical sensitization so that high sensitivity can be achieved but this method also causes deterioration with respect to the fog.